Lithium secondary batteries which are a type of commercially viable nonaqueous electrolyte secondary batteries have come into widespread use. Recently, lithium secondary batteries have attracted much attention not only as small-size batteries for use in portable electronic devices but also as large-capacity devices for installation in vehicles and electric power storage. With such trends, the requirements for safety, cost, battery life, etc., have become ever more stringent.
The main components of a lithium secondary battery are a positive electrode, a negative electrode, an electrolyte, a separator, and an outer casing. The positive electrode is constituted by a positive electrode active material, a conductive material, a current collector, and a binder (binding agent).
In general, layered transition metal oxides, a representative example of which is lithium cobaltate (LiCoO2), are used as the positive electrode active material. However, layered transition metal oxides are likely to cause oxygen desorption at a relatively low temperature of about 150° C. in a fully charged state, and this oxygen desorption may lead to a thermal runaway reaction in the battery. Accordingly, if a battery that contains such a positive electrode active material is used in portable electronic devices, accidents such as batteries generating heat and setting on fire may occur.
Thus, there is high anticipation for lithium-containing composite oxides, such as lithium iron phosphate (LiFePO4) having an olivine structure, that have a stable structure and do not release oxygen under abnormal conditions, and are safer than LiCoO2. Lithium iron phosphate does not contain cobalt whose abundance in the Earth's crust is low and thus has an advantage that it is relatively inexpensive. Another advantage of lithium iron phosphate is that it is structurally more stable than layered transition metal oxides.
However, when lithium iron phosphate is used as a positive electrode active material, the discharge capacity decreases significantly with repeated charge-discharge cycles and there is a disadvantage that the battery life is short. This is because the positive electrode active material undergoes significant expansion and contraction due to lithium insertion/extraction during charging and discharging and gradually physically detaches from the current collector and the conductive material as the number of cycles increases, resulting in breaking of the positive electrode active material structure, an increase in amount of active materials not contributing to charging and discharging, and a decrease in discharge capacity. To address this issue, studies have been made on methods for suppressing expansion and contraction of the positive electrode active material by using a lithium-containing composite oxide having a lithium iron phosphate base structure subjected to element substitution (e.g., refer to PTL 1 and PTL 2).